The subject matter of this disclosure is generally related to fabrication of an integrated computational element (ICE) used in optical analysis tools for analyzing a substance of interest, for example, crude petroleum, gas, water, or other wellbore fluids. For instance, the disclosed ICE fabrication uses in-situ spectroscopy performed in step-scan mode in combination with lock-in or time-gated detection for monitoring the ICE fabrication.
Information about a substance can be derived through the interaction of light with that substance. The interaction changes characteristics of the light, for instance the frequency (and corresponding wavelength), intensity, polarization, and/or direction (e.g., through scattering, absorption, reflection or refraction). Chemical, thermal, physical, mechanical, optical or various other characteristics of the substance can be determined based on the changes in the characteristics of the light interacting with the substance. As such, in certain applications, one or more characteristics of crude petroleum, gas, water, or other wellbore fluids can be derived in-situ, e.g., downhole at well sites, as a result of the interaction between these substances and light.
Integrated computational elements (ICEs) enable the measurement of various chemical or physical characteristics through the use of regression techniques. An ICE selectively weights, when operated as part of optical analysis tools, light modified by a sample in at least a portion of a wavelength range such that the weightings are related to one or more characteristics of the sample. An ICE can be an optical substrate with multiple stacked dielectric layers (e.g., from about 2 to about 50 layers), each having a different complex refractive index from its adjacent layers. The specific number of layers, N, the optical properties (e.g. real and imaginary components of complex indices of refraction) of the layers, the optical properties of the substrate, and the physical thickness of each of the layers that compose the ICE are selected so that the light processed by the ICE is related to one or more characteristics of the sample. Because ICEs extract information from the light modified by a sample passively, they can be incorporated in low cost and rugged optical analysis tools. Hence, ICE-based downhole optical analysis tools can provide a relatively low cost, rugged and accurate system for monitoring quality of wellbore fluids, for instance.
Errors in fabrication of some constituent layers of an ICE design can degrade the ICE's target performance. In most cases, deviations of <0.1%, and even 0.01% or 0.0001%, from point by point design values of the optical characteristics (e.g., complex refractive indices), and/or physical characteristics (e.g., thicknesses) of the formed layers of the ICE can reduce the ICE's performance, in some cases to such an extent, that the ICE becomes operationally useless. Complex refractive indices and thicknesses of layers of the ICEs being fabricated are determined by performing in-situ measurements during the ICE fabrication. The determined complex refractive indices and layer thicknesses of the formed layers of the ICEs within the fabrication batch are used to adjust forming of remaining layers of the ICEs based on comparisons between determined values of complex refractive indices and layer thicknesses of the fabricated ICEs' layers and their respective target values. Those familiar or currently practicing in the art will readily appreciate that the ultra-high accuracies required by ICE designs challenge the state of the art in thin film measurement techniques. In-situ measurements used for monitoring the ICE fabrication includes spectroscopy for acquiring spectra of formed ICE layers. Conventionally, the spectra are acquired by continuously scanning a wavelength selecting element of a spectrometer to provide spectrally different instances of probe-light. Spectra, acquired using this conventional acquisition mode, typically are affected from noise contributed by various noise sources present in the ICE fabrication environment.
Like reference symbols in the various drawings indicate like elements.